The invention pertains to a method a device for heat-sealing multiple plies of a laminate, wherein the laminate comprises a carrier layer of electrically nonconductive material and a sealing layer of thermoplastic material on at least one surface of the laminate.
For packages containing liquid food products, laminates comprising a carrier layer of electrically nonconductive material, especially paper or cardboard, and a sealing layer of thermoplastic material, especially a thermoplastic such as polyethylene or polypropylene, are frequently used as packaging material. The thermoplastic material is liquid-tight and resistant to fat-containing substances and acids. In addition, the thermoplastic material can be heat-sealed by placing opposing sealing layers of thermoplastic material against each other and by welding them together by the application of heat in a sealing region at the same time that multiple plies of the laminate are being pressed together.
The heat for the heat-sealing process is conducted from the outside, through the plies of the laminate, to the inner sealing layers. Because the packages must be sealed very quickly in modern packaging machines, it can be difficult to supply enough heat.
If the laminate comprises a metal layer, especially a layer of aluminum, the heat for heat-sealing can be generated by induction, in that coils are incorporated into the pressing bars which compress the plies of the laminate; these coils induce electrical currents in the metal ply of the laminate and thus heat it. The heat is carried by conduction from the heated metal ply to the adjacent plies of thermoplastic material, which is melted by the heat and thus converted to a fluid state.
To an increasing extent, laminates for packages are being used which, even though they do not have a metal layer, are gas-tight and have flavor-preserving properties. There is therefore a need to generate heat effectively in the laminate for the heat-sealing process even in the absence of a metal layer. DE 23 24 810 proposes that the heat be generated by the dielectric losses which occur when the laminate is exposed to a high-frequency alternating electric field on the order of 300-600 MHz in the sealing region. The lost heat is based in particular on an orientation polarization of molecules with dipole properties in the electrically nonconductive material of the carrier layer of the laminate. In addition to the orientation polarization, ions and electrons also become polarized in the alternating electric field.
So that they can be heated, the plies of the laminate must be exposed to the high-frequency alternating field between an electrode and a counterelectrode in the sealing region. The two electrodes form a capacitor, wherein the electrically nonconductive material of the laminate forms the dielectric.
The heat-sealing process in a high-frequency alternating electric field is determined by the following parameters: the sealing time, the pressure applied to the plies in the sealing region, the material properties of the laminate to be sealed, the size of the facing surfaces of the electrode and counterelectrode, the gap between these surfaces, and the power level of the HF voltage supply.
The most important material properties are in particular the relative permittivity and the loss factor of the laminate. The relative permittivity of the laminate describes the extent to which the capacitance of the capacitor formed by the electrode and counterelectrode increases when the gap between the electrodes is filled by the laminate, as dielectric, instead of air. The loss factor tan a is also a physical material property of the laminate. To generate sufficient heat through dielectric losses, the loss factor tan a should be on the order of >0.01.
It is a problem for the heat-sealing process in a high-frequency alternating electric field that the number of plies of the laminate in the sealing region can vary. If, for example, beverage containers designed as gable top packages are to be heat-sealed to seal the gable after the container has been filled, the laminate of conventional package blanks has four plies in the lower part of the sealing region and as many as five plies in an overlap region, whereas the laminate has only two facing plies in the upper part of the sealing region. Because of the short sealing time required, it is possible, in the case of gable top packages of this type, that the heat-sealing is not sufficient in the lower part of the sealing region and/or that overheating occurs in the upper part of the sealing region As a result, the seal of the package can be leaky, which is not acceptable with respect to the shelf life of the contents.